The present invention relates generally to manual transmissions for motor vehicles. More particularly, the present invention relates to a multi-speed manual transmission having a double planetary gearset consisting of two simple planetary gearsets.
As is known, manual transmissions are used in motor vehicles for transmitting engine torque to the drive wheels at various speed ratios. Modernly, most manual transmissions are available in four-speed and five-speed versions since each is particularly well-suited for vehicles equipped with smaller engines. Regardless of the number of available speed ratios, the vast majority of conventional manual transmissions are of the "layshaft" type and include: an input shaft driven by the engine and having a first set of gears supported thereon; an output shaft interconnected to the drive wheels; a countershaft coupled to the output shaft and having a second set of gears supported thereon that are in constant mesh with the first set of gears; and a set of synchronizer clutches for selectively coupling one of the meshed gear pairs for transferring power through the countershaft to the output shaft at a desired gear ratio. However, while this arrangement is typically suitable for most four-speed and five-speed manual transmissions, its application to transmissions having more than five gear ratios is severely limited because of packaging constraints and the complexity of the shift system.
A problem to be addressed with compound layshaft-type transmissions having greater than five speed ratios is commonly referred to as "swap shifting", that is, where two engaged elements are released and two others become simultaneously engaged to establish a desired gear change. This arrangement is highly characteristic of heavy duty truck transmissions in which multiple speed ratios (i.e., eight, nine or fifteen speed ratios) are required. At one time, truck transmissions were equipped with two shift levers for permitting the operator to shift through all the available speed ratios. In particular, a first shift lever was used to shift a torque splitter device while a second lever was used to shift the main gearbox. This system has been modified so that today it is very common to find trucks fitted with a two-speed splitter (high range and low range) that is either hydraulically or electronically shifted.
In an effort to overcome the above-noted difficulties associated with manual layshaft transmissions, one or more planetary gearsets have been incorporated into the geartrain to provide gear shifts with only single shifting steps. While it is known to incorporate a single planetary gear assembly into a vehicle drivetrain (see, for example, U.S. Pat. No. 5,284,068 issued Feb. 8, 1994 to Frost for TRANSFER CASE WITH INTEGRATED PLANETARY GEAR ASSEMBLY and commonly assigned to the assignee of the present application) and even to incorporate a dual planetary gear assembly (see, for example, U.S. Pat. No. 4,836,053 issued Jun. 6, 1989 to Eastman and Jagani for TRANSFER CASE WITH DUAL PLANETARY GEAR REDUCTION UNIT and also commonly assigned to the assignee of the present application), the use of two simple planetary gearsets in a vehicular multi-speed transmission is an area that is relatively unexplored. One application of this combination is shown in U.S. Pat. No. 4,638,688, issued on Jan. 27, 1987, to Hiraiwa and entitled PLANETARY GEAR TRAIN FOR AUTOMOTIVE TRANSMISSION OR THE LIKE. In general, this patent is directed to a geartrain for an automotive transmission that includes two simple planetary gearsets that are operatively controlled by four clutches and two brakes. Thus, while representing an improvement over conventional layshaft arrangements, this transmission is still overly complex, expensive to manufacture, and risks the loss of considerable power between the clutching and braking operations.